Generally, an electric motor is a device that converts electrical energy into mechanical energy to cause a rotational motion, obtain rotational power from the rotational motion, and supply the rotational power.
So as to increase the output of the electric motor and the number of rotations thereof and supply the increased results, a rated capacity of the electric motor is increased to raise ouptput thereof and an inverter is used to perform voltage and frequency modulation. In this case, however, a general electric motor should be changed into an electric motor capable of performing frequency modulation, and if the rated capacity is increased, besides, the outer shapes of a rotor and a stator become bulky, large amounts of noise and heat are generated, and the amount of power consumed is increased. Due to the use of the converter for raising the number of rotations of the electric motor, further, the installation and operation costs for the frequency conversion are undesirably increased.
Even in case of an inverter motor performing multi-step speed operations, it has a limitation in frequency modulation, thereby making it hard to obtain the number of rotations of a given value or above. The output of motor is proportional to torque and the number of rotations thereof, and so as to obtain the number of rotations as required, accordingly, the electric motor whose rated capacity is increased and the inverter have to be used, which undesirably raising an operation cost. In case of the operation is kept at a high speed, further, noise may be generated and the electric motor may be overheated, and accordingly, there is a need to solve the problems.